Movement patterns

Hello. I''m creating a type of classic space shoot em up (think gradius, ikaruga etc..) and having some problems on figuring out how to create the enemy space ships movement. What I''m trying to do is to figure out a good way to represent how a group of enemy ships will move on the screen. For example: say that i want to have one type of enemy ships that will move in a certain way, first going down the screen a bit and then go arround in a circle then finaly go down the screen again and at the same time have a different type of enemy ships go from the side of the screen in some cool pattern... How do i do this in some easy way.. I was thinking about storing the movement in a text file and then read that and assign movement that way but have really no idea how to go about doing this... Any tips, hints or pointers are welcome